"λ" is a Greek letter, pronounced: Lambda. In physics, it is used to represent wavelength. Refers to the distance that a wave travels in a vibration period. That is, the distance between two adjacent points with a phase difference of 2π along the propagation direction of waves. The wavelength λ is equal to the product of the wave velocity u and the period t, that is, λ=uT. Waves with the same frequency travel at different speeds in different media, so their wavelengths are different.

Measurement of wavelength versus wave characteristics

When discussing the propagation of elastic waves, it is assumed that the medium is continuous, because when the wavelength is greater than the distance between molecules in the medium, countless molecules in the medium are vibrating continuously within a wavelength distance, which looks like a continuous medium from a macro perspective; But if the frequency of the wave source is extremely high, the wavelength is extremely small.

When the wavelength is less than or equal to the order of magnitude of the distance between molecules, there are no other molecules between two molecules about one wavelength apart, so the medium can no longer be considered continuous and elastic waves can no longer propagate. The distance between molecules in high vacuum is extremely large, which is also the reason why sound waves cannot propagate.